1. Field of the Invention
The present invention relates to a pressure support system and to a method of using such a system, and, more particularly, to a pressure support system and method in which a parameter associated with the fluid flow in a patient circuit, such as the magnitude of flow, direction of flow, or volume of fluid passing through the patient circuit, are monitored based on a characteristic of a signal used to control a motor in a pressure generating portion of the pressure support system.
2. Description of the Related Art
It is well known to treat a breathing disorder, such as obstructive sleep apnea (OSA), with a pressure support device, such as a continuous positive airway pressure (CPAP) device. A CPAP device delivers a flow of fluid to the airway of the patient throughout the patient's breathing cycle in order to “splint” the airway, thereby preventing its collapse during sleep. The term “fluid” as used herein refers to any gas, including a gas mixture or a gas with particles, such as an aerosol medication, suspended therein. Most commonly, the fluid delivered to a patient by a pressure support system is pressured air. An example of such a CPAP device is the REMSTAR® and SOLO® family of CPAP devices manufactured by Respironics, Inc. of Pittsburgh, Pa.
It is also known to provide a bi-level positive pressure therapy in which the pressure of fluid delivered to the patient's airway varies or is synchronized with the patient's breathing cycle to maximize the therapeutic affect and comfort to the patient. An example of a pressure support device that provides “bi-level” pressure support, in which a lower pressure is delivered to that patient during the patient's expiratory phase than during the inspiratory phase, is the BIPAP® family of devices manufactured and distributed by Respironics, Inc. of Pittsburgh, Pa. Such a bi-level mode of pressure support is taught, for example, in U.S. Pat. No. 5,148,802 to Sanders et al., U.S. Pat. No. 5,313,937 to Zdrojkowski et al., U.S. Pat. No. 5,433,193 to Sanders et al., U.S. Pat. No. 5,632,269 to Zdrojkowski et al., U.S. Pat. No. 5,803,065 to Zdrojkowski et al., and U.S. Pat. No. 6,029,664 to Zdrojkowski et al., the contents of each of which are incorporated by reference into the present invention.
It is further known to provide an auto-titration positive pressure therapy in which the pressure provided to the patient changes based on the detected conditions of the patient, such as whether the patient is snoring or experiencing an apnea, hypopnea, or upper airway resistance. An example of a device that adjusts the pressure delivered to the patient based on whether or not the patient is snoring is the VIRTUOSO® CPAP family of devices manufactured and distributed by Respironics, Inc. An example of a pressure support device that actively tests the patient's airway to determine whether obstruction, complete or partial, could occur and adjusts the pressure output to avoid this result is the TRANQUILITY® Auto CPAP device, also manufactured and distributed by Respironics, Inc. An exemplary auto-titration pressure support mode is taught, for example, in U.S. Pat. Nos. 5,203,343; 5,458,137 and 6,087,747 all to Axe et al., the contents of which are incorporated herein by reference. A further example of an auto-titration pressure support device that actively tests the patient's airway to determine whether obstruction, complete or partial, could occur and adjusts the pressure output to avoid this result is the TRANQUILITY® Auto CPAP device, also manufactured by Respironics, Inc. This auto-titration pressure support mode is taught in U.S. Pat. No. 5,645,053 to Remmers et al., the content of which is also incorporated herein by reference.
In each of these pressure support systems, it is often desirable to measure the flow of fluid to or from the patient, the volume of fluid delivered to or received from the patient, or both with a relatively high degree of accuracy. One reason for doing so is to monitor whether the patient is using, i.e., complying, with a prescribed pressure support therapy. If the pressure support device is operating, yet there are no variations in the flow of fluid delivered to the patient, which would normally be present as a result of patient breathing, it can be assumed that the patient is not actually receiving the flow of fluid. This may occur, for example, if the patient is not wearing the patient interface device that communicates the flow of fluid to his or her airway so that he or she is not receiving the therapeutic benefits of the pressure support treatment.
In a bi-level pressure support device it is desirable to monitor the flow of fluid delivered to the patient via a patient circuit to detect when the patient is spontaneously changing from the inspiratory phase to the expiratory phase of the breathing cycle and vice versa. Proper detection of transitions between the inspiratory phase and the expiratory phase are necessary in order to trigger the pressure support device at the end of the patient's expiratory phase and to cycle the device at the end of the inspiratory phase.
In an auto-titration pressure support system, monitoring the flow of fluid, the volume of fluid, or both is done to detect disordered breathing. For example, monitoring the flow of fluid in the patient circuit can be used to determine whether the patient is experiencing snoring, apnea, hypopnea, cheynes-stokes breathing, or other breathing abnormalities, as well as normal breathing patterns.
Various approaches for measuring the flow of fluid, the volume of fluid, or both in the patient circuit portion of a pressure support device are well known. For example, it is known to use a pneumatach flow meter placed directly in the patient circuit to measure the flow of fluid in the patient circuit. Examples of a conventional flow or volume measuring techniques are taught, for example, in U.S. Pat. No. 4,083,245 to Osborn; U.S. Pat. No. 4,796,651 to Ginn et al.; U.S. Pat. No. 5,038,621 to Stupecky; and U.S. Pat. No. 5,743,270 to Gazzara et al.
Because this flow/volume measurement techniques requires placing a flow element in the patient circuit, as well as providing a mass flow or pressure sensor associated with the flow element, it increases the cost of the pressure support system as well as its complexity. In some direct flow measurement devices, the flow element is specifically designed to provide the desired flow measurement capabilities, which also increases the cost and complexity of the flow meter design. Furthermore, the flow element is a flow restriction in the patient circuit, thereby decreasing the efficiency of the pressure support system.
Other techniques for sensing the flow of fluid communicated between a patient by a pressure support system are also known. For example, U.S. Pat. No. 5,443,061 to Champain et al. discloses a pressure support device that is capable of detecting variations in a drive current supplied to a motor that drives a blower in the pressure generating system. The system in Champain adjusts the drive current to modulate the pressure of the fluid at a mask in response to the patient's breathing cycle. U.S. Pat. No. 5,715,812 also to Deighan et al. discloses a current monitor coupled with the blower motor of a medical ventilator. The current monitor determines whether the patient is breathing into the patient circuit based on the current provided to the motor. Finally, U.S. Pat. No. 5,740,795 to Brydon discloses a pressure support system that senses the current or the power (voltage×current) provided to an electric motor to determine when a patient transitions between the inspiratory or expiratory phases of a breathing cycle. Brydon further teaches that the actual, volumetric flow of patient respiration can be determined from the measured current or power provided to the electric motor.
The monitoring systems taught by the patents noted above, however, do not quantitatively measure the flow of fluid or the volume of fluid in the patient circuit based on the monitored parameters unless they also include separate components for sensing at least the current or power provided to the motor. That is, the devices taught in these patents cannot determine the actual (quantitative) value of the flow of gas in the patient circuit or the quantitative volume of gas passing therethrough unless they include additional components to detect motor current or power. Those skilled in the art can appreciate that the use of such current sensors increases the cost and complexity of the pressure support system.
In summary, disadvantages of the above-described conventional devices for monitoring fluid flow/volume in a pressure support system depend upon the use of external sensors, such as motor current sensors, pressure transducers, heat sensors, position sensors, pressure switches, and the like, to provide an indication of the patient's interaction with the medical ventilator. While indirect flow sensing devices may be able to distinguish between patient inspiration and expiration or monitor patient usage, they cannot quantitatively measure the flow or volume of fluid delivered to or received from the patient unless they include a motor current sensing system or other conventional flow sensor. Of course, this additional current or flow sensor and associated circuitry adds cost and complexity to such devices.